Cobra 29 LTD Classic, no output

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big mike 8113
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Cobra 29 LTD Classic, no output

Post by big mike 8113 » Tuesday 11th Jul 2006, 18:43

I have aCobra 29 LTD Classic on the binch, it has no output power. The receive works fine. The P.A. cercut works with nice loud audio. When I key the mic. in the CB mode the green light turns to red like it is supposed to, just no output carrier or modulation. I changed the drive and final transistors, IC-4 poweramp, And TR-3 x-mit/rec. regulator, and a couple misc. capasitors. Still no output. I could sure use some help on this one. I tryed using a voltage pinout chart I have, to check through the circuts, it didnt seem to match with any thing.
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Rick
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Cobra 29

Post by Rick » Tuesday 11th Jul 2006, 21:59

Stock fault IC3 TA7310P.

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Cobra 29 no output

Post by big mike 8113 » Wednesday 12th Jul 2006, 17:57

I changed out IC-3 with a new TA7310P, still no output power. Thank you Rick for your input, but still surching for the little Gremlin.
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Rick
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Cobra 29 LTD Classic, no output

Post by Rick » Wednesday 12th Jul 2006, 21:50

Ever time I have had a 29 LTD in for repair with on TX is has been the IC3 TX mixer. I think it would be best to check what is going on around the mixer, check if 10.245MHx is present at pin 4 (IC3) and check if 16Mhz plus this will depend on which channel your are on at pin 1 (IC3), the two frequency are mixed to give the output at pin 9 which should be 27MHz plus this will depend on which channel your are on, the output is taken to the Buffer transistor TR16 if you check the collector TR16 you should have the output frequency and the RF is about 0.50mV at this point. If all is ok to this point the rest is easy.

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Post by Jonathan_lemoine » Monday 12th Nov 2007, 11:56

I am also having the exact same problem (may be a different cause). If you have any luck finding that gremlin, I would like to fix my radio too.
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Post by Jonathan_lemoine » Tuesday 13th Nov 2007, 8:59

My ic3 is good, my driver is good and my final is good. what would i check next?
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Cobra 29

Post by captian_radio » Wednesday 14th Nov 2007, 4:11

Jonathan , you have one of the best tech,s in Rick helping you, do exactly as he says and you will get your 29 up and running.Awhile back he helped me with a tough problem and got it running again,tnx again Rick.
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Post by Jonathan_lemoine » Wednesday 14th Nov 2007, 13:27

Yeah I am beginning to think that Rick could build a CB from scratch and improve on many manufacturers designs. A genius. And if he had my radio, it would have been fixed 20min after he looked at it. But I have to make do with my knowledge and rely on people on the net because the there is nobody within a few hundred miles that knows how to fix a cb.
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Post by frogman296 » Wednesday 14th Nov 2007, 14:58

i am in the same boat john...don't feel bad...

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Rick
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Re: Cobra 29 LTD Classic No TX.

Post by Rick » Thursday 15th Nov 2007, 1:08

First check the VCO voltage at TP1, Let us know what the voltages is at TP1 in TX on channel 1 and on channel 40?

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Post by Jonathan_lemoine » Thursday 15th Nov 2007, 13:13

Sorry Rick, but TP1 is not not used in my radio. My radio is actually a 29 nw st. Which as far as know is pretty much identical to the LTD. Or it may even be a LTD NW ST. However, IC1 has 29 LTD printed on it.
With that said if TP1 is supposed to be empty then it has no voltage, however, I have voltage up to jp32. 7.7 volts when the mike is keyed. That may or may not be much help to you. Anyway I am not getting any voltage to R58.
Is there anything else I can test for you?
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Re: Cobra 29 LTD Classic No TX.

Post by Rick » Thursday 15th Nov 2007, 13:51

On your radio it is TP2, see diagram.

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Jonathan_lemoine
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Post by Jonathan_lemoine » Friday 16th Nov 2007, 7:29

RX - 1.62V
TX - 2.26V
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Re: Cobra 29 LTD Classic No TX.

Post by Rick » Friday 16th Nov 2007, 10:24

Rick wrote:First check the VCO voltage at TP1, Let us know what the voltages is at TP1 in TX on channel 1 and on channel 40?

Rick.
Check on channel 1 and 40 in TX

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Post by Jonathan_lemoine » Friday 16th Nov 2007, 11:40

Sorry. Channel 1 - 2.26V, channel 40 - 3.01V
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Re: Cobra 29 LTD Classic No TX.

Post by Rick » Friday 16th Nov 2007, 12:16

Voltages you have given are OK. Next you need to check IC3 in the TX mode Pin 1 should be 2.6 volts, pins 2 and 3 not used, pin 4 = 2.6 volts, pin 5 = 0, pin 6 = 7.5 volts, pin 7 = 2 volts, pin 8 = 4.8 volts, pin 9 = 7.6 volts. Lets know how you get on.

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Post by Jonathan_lemoine » Friday 16th Nov 2007, 12:41

YEP. All voltages are correct.
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Re: Cobra 29 LTD Classic No TX.

Post by Rick » Friday 16th Nov 2007, 20:50

What are the voltages on TR14, TR15, and TR16 in TX mode.

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Post by Jonathan_lemoine » Saturday 17th Nov 2007, 7:36

TR16 B=0.75 C=7.7 E=0.35
TR15 B=0 C=13 E=0.32 (I think these are the right pins, I might have E and B switched.)
TR14 B=0 C=13.2 E=0

I know there is a problem here somewhere. There is a 2.7ohm resistor at R58. This is a common mod and I think that's why I have voltage at E on TR15. But I don't quite know yet why I have nothing at B on TR14. I have the block diagram and circuit diagram for a 29 ltd st. also the service manual so I have some idea on what's in the circuit. but I'm not sure what to check next. I think the only things that are left are a handful of caps and resistors.
I'm sure you are getting close and have a pretty good idea what's up. :D
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Re: Cobra 29 LTD Classic No TX.

Post by Rick » Saturday 17th Nov 2007, 8:41

You will not see any base voltage once you have past TR16 due to the transistors being switch on 27 MHz. You need a scope or RF probe to continue faulty finding.

You could try re-tuning L21 to L17 but need a scope to do this, its not good just guessing.

However, I think you have a bad driver or final or a badly tuned PA stage.

I think its time to test the transistors or just replace them.

Rick.
Last edited by Rick on Saturday 17th Nov 2007, 11:19, edited 1 time in total.

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Post by frogman296 » Saturday 17th Nov 2007, 9:13

hmmm...if we only had an oscope...this leads me to believe that one would be a great investment

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Post by Jonathan_lemoine » Saturday 17th Nov 2007, 10:31

Hi Rick. Thanks for all your help. I really didn't think it was the driver but sure enough it was. now my radio is pushing 20+ watts. Thanks again. I learned a lot in the last few days.
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Re: Cobra 29 LTD Classic, no output

Post by RFEverywhere » Thursday 29th May 2014, 0:47

I know this is a very very VERY old thread but I came across it while trying to figure out what the issue is with my Cobra 29 Classic and Cobra 29LX.

I have been going crazy trying to figure out what the problem may be. Both seem to not transmit, however the transmit light comes on and I also get some sort of signal on nearby receivers (several inches to feet). However on my power meter I get absolutely nothing. And on the nearby receivers, I don't get any audio at all whatsoever.

In regards to the 29LX:

I have tried all of the above and the only thing out of the ordinart is 1. On TR15 & TR14 I do not have 12.xxVDC on receive and they don't match what is in the Cobra Voltage Checksheet either on transmit. 2. My voltages are just a little bit different than the voltages above on the other TP and IC pins but only by +/- 0.1V - 0.5V. The frequencies seem to be right on but just a little on the unstable side. But that could simply be my POS cheap frequency counter.

I will try to check R58 a little later and will try to swap out both the 2078 and the driver and see what happens.

I'm really not sure what may have happened between the last time that I used it and two weeks ago when I dug it out to try and put it on eBay.

In regards to the 29LTD CLASSIC:

I had played around with it several days ago and tried replacing the the 2078 and the driver and nothing. No improvements at all. I did not and have not yet gone through the above steps on this particular unit.

I should mention that this (in fact both units) is a brand new unit that I unsuccessfully tried to do an IRF520 upgrade on and was only getting about 1 watt out. Then all of the sudden I stopped getting any power out on the power meter and decided to go back to factory layout and restart all over again and came to be where I am now. I also noticed that the antenna warning light is constantly on regardless of whether I am transmitting or not.

I do have all the necessary test equiptment (scope, AF generator, RF generator, DMM, frequency counter, etc....)

I'm not sure if Rick is still here on the forum or not but I sure could use a helping brain/hand.

I look forward to receiving and will appreciate all comments, pointers, replies, and all criticisms are welcome as well.

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Re: Cobra 29 LTD Classic, no output

Post by RFEverywhere » Thursday 29th May 2014, 0:53

I have been going crazy trying to figure out what the problem may be. Both seem to not transmit, however the transmit light comes on and I also get some sort of signal on nearby receivers (several inches to feet). However on my power meter I get absolutely nothing. And on the nearby receivers, I don't get any audio at all whatsoever
Just wanted to add that both PA and Talkback work perfectly fine and I can get audio out but just no modulation when transmitting. Just hear silence on the nearby receivers.

Thank you again.

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Re: Cobra 29 LTD Classic, no output

Post by lbcomms » Friday 13th Jun 2014, 1:31

RFEverywhere wrote:I know this is a very very VERY old thread but I came across it while trying to figure out what the issue is with my Cobra 29 Classic and Cobra 29LX
Old thread is something of an understatement :lol:
the transmit light comes on and I also get some sort of signal on nearby receivers (several inches to feet)
on the nearby receivers, I don't get any audio at all
That tells you the DC switching is working OK, and that the transmit mixer or doubler is at least partially functional. You just need to find out where it is losing its signal.

The reason there is no audio is because the audio is added to the carrier at the driver and final, and your TX signal is not getting that far.

Your Tx 27Mhz signal is being lost somewhere between the mixer or doubler and the driver. With a schematic diagram and a scope (or a short piece of wire and a second radio tuned to the same channel if you don't own a scope) you shouldn't have any trouble finding the trouble.

Last one we had with no TX was its bigger SSB cousin (Cobra 142). One of the bandpass cans between the mixer and driver went open circuit internally, killing transmit.
On AM is just threw a small carrier with no audio to a nearby receiver.

Good luck...

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